Grey is rangi ya kijivu, the colour of ash. Ash is jivu, so the ki- is already something that has been added on. Haven't figured out why yet, but green, kijani (rangi ya kijani, the colour of leaves), is another.
I'm working on a theory, but if anyone who knows turns up in the meantime, hopefully they can elaborate.
Edit to add: I'm being stupid. "Colour of X" formations, X is a noun, not an adjective, so it won't take agreement from another noun.